There are a few instances where caster lb is preferred like in Garuda HM against both sisters and Garuda. I can't think of any times where it's absolutely required but there are fights where the caster lb is the one expected to be used because of the fight mechanics. For example, Titan extreme, the lb is generally saved for the caster to use on the adds. Again not absolutely necessary but it makes the fight a lot easier.
The fights are designed to utilize certain types of limit breaks in some fights more then others. Another example is the only time a tank lb is the favored one is in ultima where the bombs will wipe everyone unless the tank uses lb to have the party survive the explosions.
The times will come but it's all 50+ content. Pre-50 dungeons just use your best judgment, if there are adds in a boss fight and no one is lbing, go for it